The invention relates to the fuel management control system of internal combustion engines. More particularly, the invention relates to a method and an apparatus for determining the operational readiness of an oxygen sensor for measuring the exhaust gas composition. The fuel management control system could be of any suitable type, using carburetion or fuel injection. In fuel injection, for example, control pulses would be applied to electromagnetically actuated fuel injection valves in synchronism with the crankshaft rotation and in dependence on the engine rpm and on the air flow rate. When mixture compressing, internal combustion engines are supplied with fuel, the fuel must be so adapted to the aspirated air quantity that the combustion process is complete and does not lead to a loss of power, nor excess fuel, which would produce an excessive amount of toxic exhaust gas constituents.
An attempt is thus made to supply a fuel-air mixture to the combustion chambers which is in the stoichiometric ratio (air no. .lambda.=1) or in which there is excess air and which, in any case, can be adjusted to preselected values. If the engine is operated in the domain of excess air, i.e., at its lean running limit, it is particularly easy to reduce exhaust gas constituents and to conform to the ever more rigid requirements for atmospheric purity.
The amount of fuel which must be supplied to the engine is determined on the basis of known criteria, among which are, principally, the engine rpm and the air flow rate. However, it may be desirable to check the air-fuel mixture and to correct its ratio if the desired values are not maintained. This purpose can be attained by employing a closed loop control process using a known so-called .lambda.-sensor. Such a .lambda.-sensor is used nowadays in internal combustion engines and is capable to generate a signal related to the composition of the exhaust gas indicating certain values of the air number .lambda.. In particular, the present day .lambda.-sensor generates a relatively low positive potential when the engine in which it is used operates with a rich mixture, indicating a value .lambda. less than 1. If the mixture used is lean, the output signal from the .lambda.-sensor is effectively zero. When the fuel-air mixture changes composition and passes through the value .lambda.=1, the signal from the .lambda.-sensor undergoes an abrupt change in the manner of a step function and its use is thus limited substantially to the region in the vicinity of .lambda.=1, at least with great precision. The change from a very low output voltage to what is essentially the maximum output voltage is very abrupt. However, a .lambda.-sensor of this type may be used not merely to adjust the fuel quantity fed to the engine, but to control it in a closed loop by making the internal combustion engine itself the controlled system and by letting the fuel management control system constitute the actual controller which is supplied with the output signal of the .lambda.-sensor as the actual value of the control variable.
When a .lambda.-sensor is used in a closed control loop for operating a fuel management control system, a difficulty arises from the fact that the sensor may not always be in normal operating condition, for example, when it is cold, when the engine itself is cold, or during hot starting after a short period of rest. Sometimes the electrical connections leading to the sensor may be broken or short-circuited. In all of these cases, of which that of a cold sensor probably occurs most often, the control process is defective because when cold, the .lambda.-sensor does not generate a usable output signal.